1. Field of the Invention
The present invention relates to a pixel driving circuit, especially a pixel driving circuit of an active matrix organic light emitting diode (AMOLED).
2. Description of the Prior Art
Due to their slim shapes, low power consumption and low radiation, liquid crystal displays (LCDs) are widely applied in mobile electronic devices such as notebooks, monitors, and PDAs (personal digital assistants). Besides, the organic light emitting diode (OLED) display can be operated without a backlight source and color filters, and has a slimmer shape and better performance in color, thus the OLED display is also widely used.
Please refer to FIG. 1, which shows a prior art pixel driving circuit 100 applied in an OLED display. As shown in FIG. 1, the pixel driving circuit 100 comprises a first switch M1, a second switch M2, a capacitor C0 and an OLED D1. The first end of the first switch M1 is used to receive the data signal Data, and the control end of the first switch M1 is used to receive the scan signal Scan and to turn on or off the first switch M1 according to the scan signal Scan. The capacitor C0 is coupled to the second end of the first switch M1 for storing the data signal Data to keep the voltage level of the pixel grey level. The control end of the second switch M2 is coupled to the second end of the first switch M1, and the first end of the second switch M2 is coupled to the first voltage source Vdd. The first end of the OLED D1 is coupled to the second end of the second switch M2, and the second end of the OLED D1 is coupled to the second voltage source VSS. When the first switch M1 is turned off, the second switch M2 will be turned on or off according to the voltage level of the data signal Data stored in the capacitor C0, to control the amount of the current I1 flowing through the second switch M2 and the brightness of the OLED D1.
Therefore, the second switch M2 plays an important role as a switch to accurately control the amount of the current I1. If the second switch M2 has a threshold voltage shift, the amount of the current I1 will change accordingly, causing the display using the pixel driving circuit 100 unable to display a correct grey level. Regarding the shift of the threshold voltage, a-Si TFT has a larger threshold voltage shift than poly-Si TFT in the stress process, so that the current provided by a-Si TFT will deteriorate faster than that by poly-Si TFT.
Although there are some compensated circuits designed for solving the aforementioned problem of the pixel driving circuit 100, the number of the total components such as switches and capacitors in the circuits has to be increased. This reduces the aperture ratio of the display, and raises the difficulty to design the driving circuit of a high definition panel.
Moreover, in the prior art pixel driving circuit 100, the current I1 flows through the second switch M2 and the OLED D1. The product of the current I1 and the voltage across the OLED D1 represents the power consumption of the OLED D1. The power consumption of the OLED D1 can only be reduced by improving emission efficiency of the OLED D1. On the other hand, the product of the current I1 and the voltage across the second switch M2 represents the power consumption of the pixel driving circuit 100. This portion of the power consumption can be reduced by reducing the amount of the current I1 or reducing the amount of the voltage across the second switch M2.